Ophthalmic examinations are prevalently made for the purpose of earlier diagnoses of various diseases that are among the top reasons of lifestyle-related diseases and causes of blindness. A tomography apparatus for an eye portion such as an OCT (Optical Coherence Tomography) is expected to effectively give more adequate diagnoses of diseases since it allows to three-dimensionally observe the state of the interior of retina layers. By measuring a change in layer thickness of, for example, a nerve fiber layer or retina, and a change in layer geometry such as an unevenness of a retinal pigment epithelium from this tomogram, it is possible to quantitatively diagnose the degrees of progress of diseases such as glaucoma, macular edema, and age-related macular degeneration, and recovery levels after medical treatments. In order to quantitatively measure the thickness of these layers, a technique for detecting respective layers of a retina from a tomogram using a computer and measuring the thickness of these layers has been proposed (see Japanese Patent Laid-Open No. 2008-073099).
On the other hand, in an OCT tomogram, when measurement light is strongly reflected or absorbed by an object, an artifact caused by attenuation or omission of signals is often generated behind the object. Note that the object includes, for example, tissue such as a blood vessel and morbid portions such as an exudate and bleeding. As shown in FIG. 1A, when intensities are displayed normally in association with a depth direction (to be referred to as a z-axis direction or an A-scan direction hereinafter) of a retina, a maximum intensity appears in the vicinity of a retinal pigment epithelium 2. However, as shown in FIG. 1B, when an artifact region 5 is generated on the positive direction side of the z-axis of a retina blood vessel 4, intensities near a retinal pigment epithelium 6 in the artifact region 5 are attenuated or omitted. Therefore, it often becomes difficult to extract a layer and to measure the layer thickness and layer geometry depending on the degree of attenuation of intensities of the layer in the region where an artifact is generated. To solve this problem, a technique which extracts a blood vessel region from a surface image of an eye fundus, back-projects the blood vessel region onto an OCT tomogram, and interpolates layers in the vicinity of the back-projection region, so as to estimate a layer position in an artifact region caused by a blood vessel has been proposed (see Japanese Patent Laid-Open No. 2007-325831).